The Sing Off Would Actually Work If...
The judges didn't have an agenda, which is to end up with a) a marketable, money making act, and b) to preserve good ratings for the show. They do have this agenda, it's what the show is about, so it all becomes a bit redundant.
I can't think of a sing off where with this in mind I have been genuinely surprised by the result. The only time I was shocked was when Jedward stayed over Lucy (was that her name?) and I have to admit, in my horror at Jedward's lack of talent, that was because I forgot about b)